In operation, internal combustion engines generate frequently considerable amounts of nitrogen oxides (NOx). Especially in the case of diesel and gasoline engines used in motor vehicles, the amounts of nitrogen oxide in the untreated exhaust gas generally exceed the permissible limits, and so exhaust gas aftertreatment is needed to reduce the NOx emissions. In many engines, nitrogen oxides are reduced by the nonoxidized and/or partially oxidized constituents present in the exhaust gas, namely by carbon monoxide (CO) and uncombusted hydrocarbons (HC), with the aid of a three-way catalytic converter. Especially in the case of lean burn diesel and gasoline engines, however, this method is unavailable since the high oxygen content in the exhaust gas means that there is no or barely any reduction of NOx. Particularly in the case of lean burn gasoline engines, but also in the case of diesel engines, therefore, in a widespread method, a NOx storage catalyst (also “LNT” from “lean NOx trap”) is used, which adsorbs the nitrogen oxides present in the exhaust gas of the internal combustion engine. Once the absorption capacity of the NOx storage catalyst is exhausted, it is typically regenerated by “rich” operation (operation with excess fuel) of the internal combustion engine.
According to the region of production, mineral oil contains various proportions of sulfur. In the commercial fuels produced from mineral oil, there are accordingly residual contents of sulfur. The permissible sulfur contents for filling station fuel have been lowered significantly over the years within the scope of legal measures at least in Europe and North America. Nevertheless, the internal combustion engine, especially diesel engines, is still supplied with residual amounts of sulfur with the fuel. After combustion, this sulfur is present in the exhaust gas.
The ability of the NOx storage catalyst to function decreases with increasing operating time, which is attributable partly to contamination of the NOx storage catalyst with the sulfur present in the exhaust gas, and also to thermal aging as a result of high temperatures. Sulfur is adsorbed in the NOx storage catalyst as sulfur oxide SOx. Sulfur contamination or SOx contamination of the NOx storage catalyst can thus be mentioned synonymously in the context under discussion here. Contamination with sulfur is typically removed from the NOx storage catalyst by alternating operation with a rich and lean air/fuel ratio λ at high exhaust gas temperatures. This is also referred to as a deSOx operation. However, the high exhaust gas temperatures during such a deSOx operation lead to thermal aging of the NOx storage catalyst. This thermal aging is irreversible. The aging of the NOx storage catalyst is thus composed of irreversible thermal aging and apparent aging resulting from sulfur contamination.
A further drawback of deSOx operations is that operation with high exhaust gas temperatures and a rich air/fuel ratio increases the fuel consumption of the motor vehicle. It is therefore advisable to minimize the number of deSOx operations, both in order to minimize fuel consumption and increase the lifetime of the NOx storage catalyst. For this purpose, it is desirable to know the contamination of the NOx storage catalyst with sulfur with maximum accuracy.
Aging of NOx storage catalysts is manifested in various effects. Firstly, the absolute NOx storage capacity of a NOx storage catalyst is reduced. Secondly, there is an increase in the NOx slip in the course of flow through the NOx storage catalyst. The “NOx slip” is the proportion of NOx present in the exhaust gas which is not adsorbed by the NOx storage catalyst. More particularly, the NOx slip, even though the storage capacity of the NOx storage catalyst is not exhausted, increases with increasing aging. General monitoring of the ability of the NOx storage catalyst to function is increasingly desired, in order to avoid noncompliant pollutant emissions during motor vehicle operation.
One method of diagnosis of NOx storage catalysts is based on the comparison of the air/fuel ratio signals λ in the exhaust gas upstream and downstream of the NOx storage catalyst during the regeneration of the NOx storage catalyst. Such air/fuel ratio signals can be ascertained, for example, by conventional lambda probes.
In order to initiate regeneration of the NOx storage catalyst, the internal combustion engine is switched to the mode of operation with λ<1. A lambda probe arranged upstream of the NOx storage catalyst detects this within the scope of the flow rate of the exhaust gas from internal combustion engine to measurement site and from the intrinsic dynamics with no delay. However, a lambda probe downstream of the NOx storage catalyst does not perceive the “enriching” of the air/fuel ratio λ, on commencement of the regeneration. Only once the NOx and oxygen adsorbed in the NOx storage catalyst is converted for the most part does a lambda probe arranged downstream of the NOx storage catalyst “see” the “enriching”.
For example, DE 102012218728A1 utilizes the comparison of the air/fuel ratio signals upstream and downstream of the NOx storage catalyst to assess the aging thereof. DE 102012218728A1 also integrates the two signals and compares the value of the integrals in order to achieve a more meaningful conclusion with low enrichment of the air/fuel mixture.
Further methods of diagnosis of NOx storage catalysts are based on the comparison of NOx contents upstream and downstream of the NOx storage catalyst. Such NOx contents can be ascertained, for example, with NOx sensors. The NOx contents upstream and downstream of the NOx storage catalyst can be used to ascertain NOx slip. With increasing aging of a NOx storage catalyst, there is an increase in the NOx slip.
However, the inventors herein have recognized issues with the above approaches. While the approaches may enable monitoring of the aging of the NOx storage catalyst, the approaches do not address NOx storage catalyst performance degradation resulting from sulfur load. Accurate knowledge of the sulfur loading or sulfur state may aid in reducing desulfurization frequency.
Accordingly, a method of ascertaining a state of aging of a NOx storage catalyst of a motor vehicle with an internal combustion engine is disclosed. In the method, a first proportion of aging of the NOx storage catalyst caused by thermal aging and a second proportion of aging of the NOx storage catalyst caused by sulfur loading are determined and used to ascertain an overall state of aging of the NOx storage catalyst. The second proportion may trigger a desulfurization operation.
There is no known method to date of ascertaining the state of aging of NOx storage catalysts which, like the method of the disclosure, separately ascertains the proportion of the aging resulting from sulfur loading and the proportion of the aging resulting from thermal aging. Through the separate ascertainment of the proportions of aging, it is firstly possible to optimize the frequency of the deSOx operations and secondly to achieve an unambiguous assessment of the irreversible damage to the NOX storage catalyst. By contrast, this is not possible by the methods described to date.
The method of the disclosure especially allows a restriction in the deSOx operations to the minimum degree necessary for the desulfurization. This advantageously achieves low fuel consumption of the motor vehicle of the disclosure and low thermal aging of the NOx storage catalyst installed in the motor vehicle of the disclosure.
Moreover, the method of the disclosure allows an advantageously exact statement as to the thermal state of aging of the NOx storage catalyst effectively at any time. Existing diagnosis methods enable such statements only after a complete deSOx operation and are less exact, which promotes misdiagnoses.
The method of the disclosure for determining the proportions of the aging of the NOx storage catalyst may be configured such that at least two diagnosis methods are used. In one example, a method of the disclosure is configured such that at least one of the diagnosis methods is sensitive to sulfur loading of the NOx storage catalyst by a first amount and at least one is sensitive to sulfur loading of the NOx storage catalyst by a second, different amount. For example, the first amount may be greater than the second amount, such that one of the diagnosis methods is more sensitive to sulfur loading than another of the diagnosis methods.
The diagnosis method which is less sensitive to sulfur loading may, for example, be a method which compares at least one air/fuel ratio signal upstream with at least one air/fuel ratio signal downstream of the NOx storage catalyst. More particularly, the diagnosis method which is less sensitive to sulfur loading of the NOx storage catalyst may be a method which compares at least one air/fuel ratio signal upstream of the NOx storage catalyst integrated over a period of time with at least one air/fuel ratio signal downstream of the NOx storage catalyst integrated over the same period of time. Such air/fuel ratio signals, also called lambda signals, may be ascertained by conventional lambda probes. However, they may also in some cases be obtained from models or calculated from other signals. For example, the lambda signal upstream of the NOx storage catalyst can be ascertained from an air mass flow and a fuel mass flow into the internal combustion engine.
Such diagnosis methods based on lambda signals ascertain the NOx storage capacity on the basis of the conversion of CO and HC in the NOx storage catalyst during the “rich” regeneration. They have low sensitivity to sulfur loading of the NOx storage catalyst.
In a method of the disclosure, the diagnosis method which is sensitive to sulfur loading of the NOx storage catalyst may be a diagnosis method which ascertains the NOx slip through the NOx storage catalyst from at least one NOx signal upstream of the NOx storage catalyst and at least one NOx signal downstream of the NOx storage catalyst. More particularly, the method which is sensitive to the sulfur loading of the NOx catalyst may be a method which ascertains the NOx slip through the NOx storage catalyst from at least one NOx signal upstream of the NOx storage catalyst integrated over time and at least one NOx signal downstream of the NOx storage catalyst integrated over time. The NOx signals used can be ascertained by conventional NOx sensors. However, they may also in some cases be obtained from models or calculated from other signals. For example, the NOx signal upstream of the NOx storage catalyst can be ascertained from an engine map-based model of the internal combustion engine.
Methods based on the direct ascertainment of the NOx slip have great sensitivity with regard to the sulfur contamination of the NOx storage catalyst. This is because these methods directly determine the amount of NOx adsorbed in the NOx storage catalyst. Potential NOx storage capacity in the NOx storage catalyst blocked by SOx is thus detected as non-existent.
A method of the disclosure may be configured such that it triggers an operation for desulfurization, also called deSOx operation as described above, according to the proportion of the aging of the NOx storage catalyst caused by sulfur loading. For instance, the NOx storage capacity of the NOx storage catalyst is reset to the value still possible at the given thermal aging by desulfurization before occurrence of impermissible NOx emissions. At the same time, however, unnecessary deSOx operations are avoided.
A method of the disclosure may also be configured such that it assesses the NOx storage catalyst as being degraded (e.g., degraded beyond the degree permissible for compliance with the emissions standards) according to the proportion of the aging of the NOx storage catalyst caused by thermal aging. Such an impermissibly damaged NOx storage catalyst generally has to be exchanged. This may be communicated to the driver by a warning message, such that the vehicle may be subjected to a repair before occurrence of impermissible NOx emissions. At the same time, misdiagnoses and unnecessary exchange of the NOx storage catalyst are avoided.
Another part of the disclosure is a monitoring unit for monitoring the state of aging of a NOx storage catalyst of a motor vehicle having an internal combustion engine during the operation of the motor vehicle. The monitoring unit is set up to execute the method of the invention for ascertaining the state of aging of the NOx storage catalyst. For this purpose, the monitoring unit may especially also be configured (e.g., executing instructions) to ascertain signals as to the exhaust gas composition upstream and downstream of the NOx storage catalyst.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.